1. Field of the Invention
The present invention relates to a chain tensioner that includes a tensioner body having a cylindrical plunger bore with an open end, a cylindrical plunger slidable within the plunger bore, and an urging unit accommodated inside an oil pressure chamber formed between the plunger bore and the rear end of the plunger such as to be able to expand and contract and to urge the plunger outward, the chain tensioner being capable of maintaining suitable tension of a chain.
2. Description of the Related Art
Chain tensioners are commonly used for maintaining suitable tension of chains. In a chain guide mechanism that slidably guides a transmission chain such as an endless roller chain passing over respective sprockets of a crankshaft and a cam shaft inside an engine room with a guide shoe, for example, it is known to urge a pivoting chain guide having the guide shoe with a chain tensioner to maintain suitable tension.
For example, as shown in FIG. 16, an endless timing chain CH passes over a drive sprocket S1 attached to a crankshaft and a pair of driven sprockets S2 and S3 attached to cam shafts in an engine room. A chain guide mechanism has a pivoting chain guide G1 and a stationary chain guide G2 for guiding this timing chain CH.
The stationary chain guide G2 is fixed in position in the engine room with two mounting shafts B1 and B2, while the pivoting chain guide G1 is attached in the engine room such as to be pivotable around the mounting shaft BO in the plane in which the timing chain CH runs.
The chain tensioner 500 maintains the tension of the timing chain CH at a suitable level as well as reduces its vibration by pressing the pivoting chain guide G1.
The known chain tensioner 500 used in such a chain guide mechanism includes, for example, as shown schematically in FIG. 17, a tensioner body 510 having a cylindrical plunger bore 511 with an open end, a cylindrical plunger 520 in the plunger bore 511 to slide against the cylindrical surface 513 of the plunger bore 511, and urging means that urges the plunger 520 outward from the plunger bore 511.
The urging means is a coil spring 540 accommodated inside a cylindrical recess 521 in the cylindrical plunger 520 and compressed between the plunger and the bottom 512 of the plunger bore 511.
Oil is supplied from an oil supply hole 514 formed in the plunger bore 511, so that an oil pressure chamber 501 formed between the plunger bore 511 and the plunger 520 is filled with oil, which oil urges the plunger 520 outward. A check valve 550 (schematically shown as a check ball) stops the oil from flowing out from the oil supply hole 514.
As the plunger 520 reciprocates, oil flows through the small gap between the plunger 520 and the plunger bore 511, and the flow resistance provides the damping effect of slowing down the reciprocal movement of the plunger 520.
In such a known chain tensioner 500, when let stand for a long time after the oil supply is stopped (in the case with an engine, after the engine is stopped), there is a time lag before oil is supplied immediately after the next start-up, and as no oil is replenished but the oil in the oil pressure chamber 501 leaks out when the plunger 520 moves back and forth, there occurs a shortage of oil in the oil pressure chamber 501. Because of this shortage of oil, the damping force of the oil does not act on the plunger 520, which may sometimes lead to vibration and abnormal noise of the timing chain C, or damage the timing chain C.
It is also known to provide an oil reservoir chamber inside the plunger configured to supply oil into the oil pressure chamber (see Japanese Patent Application Laid-open No. H09-177907 and Japanese Patent Publication No. 4376278). A constant amount of oil is retained in the oil reservoir chamber, so that, even immediately after the start-up after a long non-use time, oil remaining in the oil reservoir chamber is supplied to the oil pressure chamber, to maintain the damping force of oil for the plunger and to prevent vibration or damage of the chain.
With the technique described in Japanese Patent Application Laid-open No. H09-177907, however, most of the oil supplied from the oil reservoir chamber (oil reserve chamber 9) into the oil pressure chamber (high pressure chamber 14) leaks out from the oil pressure chamber (high pressure chamber 14) through the gap between the plunger and the plunger bore (plunger chamber 4) as the plunger reciprocates immediately after the start-up, and leaks out from the oil supply hole (supply hole 12) or from the distal end of the plunger, so that the amount of oil in the oil reservoir chamber (oil reserve chamber 9) is reduced.
It is therefore necessary for the oil reservoir chamber (oil reserve chamber 9) to have a large capacity so as to keep enough oil inside during the time lag before oil is supplied from the oil supply hole (supply hole 12) immediately after the start-up, because of which the entire tensioner could not be made shorter and could hardly be made smaller.
With the technique described in Japanese Patent Publication No. 4376278, the oil supply hole (oil supply hole 122) is arranged in an upper part and oriented diagonally forward so that at least the oil that has leaked from the oil pressure chamber (high pressure oil chamber R) through the gap between the plunger and the plunger bore is prevented from leaking through the oil supply hole (oil supply hole 122) to the outside.
However, this configuration cannot be applied to a design in which oil is supplied laterally. Universal use of the tensioner was difficult because of the restrictions on the position of the oil supply hole (oil supply hole 122) or mounting orientation of the chain tensioner.
As the oil inevitably leaks out from the distal end of the plunger, the tensioner needs to have an oil reservoir chamber with a large volume as with the technique described in Japanese Patent Application Laid-open No. H09-177907, because of which the entire tensioner could not be made shorter and could hardly be made smaller.
Another problem was that the plunger structure was made complex because of the anti-rotation mechanism that was essential to stop rotation of the plunger.
The plunger features such as the oil reservoir chamber and oil supply hole in the known technologies of Japanese Patent Application Laid-open No. H09-177907 or Japanese Patent Publication No. 4376278 were formed by machining a one-piece plunger. The production process was therefore complex because the plunger needed machining in many parts, and also the cost was increased because of material limitations.